The present invention relates to a method for removing harmful organic compounds from the aluminate liquor obtained in the production of alumina according to the Bayer process.
The starting material for the alumina production according to the Bayer process is known to be bauxite which contains larger or smaller quantities of organic compounds depending on the source of the bauxite. The alumina is extracted from the bauxite by digesting at elevated temperature and pressure with a strong solution of caustic soda. A mixture containing the bauxite and caustic soda solution is sent to the digesters, and heated with steam. The digestion temperatures varies from approximately 200.degree. C. to 245.degree. C., the alumina acts as an acid to form sodium aluminate in solution while impurities remain insoluble as a red mud. After the pressure digestion, the red mud is removed from the aluminate liquor as by decantation and filtration, and then the alumina is separated from the aluminate liquor by precipitation by cooling the liquor, adding seed crystals and agitating or stirring to crystalize out the alumina as a hydrate. The crystallized out alumina is separated from the aluminate liquor as by filtration, and the resulting thinned or spent aluminate liquor is concentrated as by evaporation and then recycled to be mixed with incoming bauxite in the digesters. During the evaporation, there is a foam formation in the aluminate liquor.
The major proportion of the organic compounds in the bauxite, which are mainly humic acid derivatives, is dissolved in the circulating aluminate liquor during the digestion of the bauxite with liquor. The dissolved organic compounds adversely influence in this form the various stages in the Bayer process. Due to the just-mentioned aluminate liquor circulation and recycling in the alumina manufacture, the organic compounds in the aluminate liquor may accumulate to such an extent that it is impossible to avoid malfunctions in the process sequence.
All of these observations have now led to the result that numerous experiments have been made during the last years with the aim of explaining the composition of the organic compounds contained in bauxite or in the aluminate liquor, respectively. For example, the publications by K. Solymar et al in Freiberger Forschungshefte, (in translation, Freiburg Research Papers), Volume 103 (1965), pages 61 to 80, and F. Matthes et al in Chem. Techn., Volume 14, pages 610 to 613, illustrate such research.
The experiments resulted in a series of important realizations. For example, evaporation conditions change as a result of foam formation beginning with a certain content of organic compounds in the aluminate liquor to the extent that controlled crystallization and precipitation of foreign salts from the aluminate liquor are no longer possible. Furthermore, the concentrated thick liquor is penetrated by foam to a different extent. As a result, faulty charges are produced for the decomposition of bauxite and this leads to a worsening of the degree of decomposition. Moreover, too high a content of organic carbon compounds in the thickeners where the red mud is removed leads to a reduction in sedimentation, thus requiring larger quantities of added flocculating agent. And the organic compounds result in worsening of the crystallizing out of the alumina by stirring, and thus an often undesirable reduction in grain size of the crystallized and stirred-out hydrate.
In addition, the poorer crystallizing out and stirring effect leads to reduced product yield with respect to the stirring volume and thus to a reduction in production capacity. Furthermore, it has been noted in liquors with higher contents of organic carbon compounds that crust formation is increased, particularly in the stirrers and in the evaporation stations where the spent liquor is concentrated.
It is also known that the above-described multitudinous difficulties are not produced by the entirety of the organic compounds which are dissolved in the aluminate liquor, but substantially by a "discoloring fraction" which probably includes sodium salts and humic acid derivatives.
It is also known that part of the organic compounds is absorbed by the red mud and is removed from the circulating aluminate liquor upon the separation of the red mud from the aluminate liquor. This proportion of organic compounds that is removed with the red mud, however, is not sufficient to prevent an increase of the organic carbon level in the circulating aluminate liquor.
In order to overcome the above-described difficulties, various procedures have been proposed in the past. In most cases, however, these processes are technically too complicated or require high operating costs. Most of these processes additionally have the decisive drawback that the reduction of organic carbon is very unselective, that is, removal or decomposition of the discoloring organic compounds is achieved only in part or not at all. Thus, for example, addition of solid sodium hydroxide according to the process disclosed in Hungarian Pat. No. 146,023 results in such a high local NaOH concentration in the aluminate liquor that only partial decomposition of the discoloring organic compounds is possible and the above-described difficulties are not overcome. Other processes that have been proposed relate to chlorination of the aluminate liquor, or the introduction of ozone-containing air, that is, to an oxidation of the organic carbon compounds. The electrochemical oxidation of the aluminate liquor and the use of adsorption processes with activated carbon are further possibilities for removing the harmful organic compounds. Finally, it is possible to remove organic compounds from bauxite before decomposition by a glow treatment. For the above-mentioned reasons, however, none of the proposed processes has been realized in practice to any significant extent.